FIG. 1 diagrammatically illustrates the demodulator section of a conventional digital cable receiver device. An analog control loop includes a tuner 11, an A/D converter 13, and wideband (analog) AGC (automatic gain control) 15. The initdemod module 17 moves the input signal spectrum centered on a small IF frequency to the DC level. The Nyquist filtering 19 folds the roll-off part of the spectrum out of the Nyquist bandwidth into the in-band, and filters interference out of the Nyquist bandwidth. A digital AGC stage 21 adjusts the amplitude of the demodulated symbols to compensate for the energy that has been filtered out by the Nyquist filter, and also to compensate for the gain of the Nyquist filter, which gain depends on the ratio between the (fixed) sampling clock frequency and the (arbitrary) symbol rate. The bandwidth of the Nyquist filter is controlled by setting the symbol rate of the receiver.
Conventionally, the carrier recovery, equalizer and FEC decoder module at 23 determines whether an input carrier signal is a digital carrier or an analog carrier. This module implements a scanning algorithm which, for example, scans all different symbol rates and QAM modes that may be present in a real-world cable network. For a digital carrier, and continuing with the aforementioned QAM example, the scanning algorithm will lock on the symbol rate/QAM mode associated with the digital carrier. However, the scanning algorithm can only identify an analog carrier by scanning through all possible symbol rate/QAM mode combinations, and failing to lock on any of the possible combinations. As a result, the time consumed exhaustively scanning across the whole frequency range can become prohibitively long.
It is desirable in view of the foregoing to provide an effective technique for differentiating between analog and digital carriers in digital cable communication receivers.
Exemplary embodiments of the invention observe the signal level in the communication channel of the digital cable receiver at different channel bandwidths, and use these observations to differentiate between analog and digital carriers, without resort to the carrier recovery scanning algorithm.